1. Field of the Invention
This invention relates to a communication system comprising at least two communication units, each having the same functional characteristics, which provides discrete addressing, privacy and protection against interfering signals within the operating wave band.
In copending application Ser. No. 754,375 filed Aug. 21, 1968, in the name of Walter Ewanus, and identified in the files of the assignee as WE Case 38,667, there is described and claimed a communication system somewhat similar to the present system utilizing at least two communication units each having characteristics common to both, such as transceiver components, so that either can serve as a transmitter or receiver. In that application, as well as in the present instance, one of the important features is the arrangement of a high speed bidirectional modulator-correlator module between the antenna and the transceiver module. For purposes of simplification, because the device referred to hereinabove as a modulator-correlator is actually capable of performing the dual function, it will be referred to as a "modulator" module when the device is operating as a part of the transmitter and will be called a "correlator" module when the device is operating as a part of the receiver unit.
In both systems phase shift keying (PSK) modulation is used to spread the spectrum and since the modulator module is bidirectional it spreads the spectrum of the transmitted signals as well as undesired received signals. This function in conjunction with digital coding provides discrete addressing privacy and protection against unwanted interfering signals. Although the communication units are illustrated as being transceiver units, it is obvious that separate transmitter and receiver components may be substituted for the transceiver components.
In the system of the previously mentioned patent application, privacy is accomplished by superimposing the intelligence modulation directly on the carrier by any type of modulation in which the amplitude of the carrier remains constant, such as in phase, angle or frequency modulation. Pseudo-noise modulation is also applied to the modulation of the carrier for the purpose of further spreading the spectrum such that practically no signal power is within the aperture of a conventional receiver not employing the special modulator correlator module employed in the units of the system. In that system, if any modulation is used which causes the carrier amplitude to vary, a conventional AM receiver, which can overcome the processing gain by virtue of its close proximity to the transmitter, can recover the modulating intelligence by detecting the amplitude of the noise spectrum.
The present invention is an improvement over the system described in said patent application in that the present invention utilizes pulse width modulation in conjunction with pseudo-noise coded PSK, modulation so that the amplitude of the carrier, from the transmitter is constant and therefore a conventional receiver, although being close enough to pick-up sufficient carrier power cannot demodulate the signal intelligence from the resultant spectrum.
2. Description of the Prior Art
While spread-spectrum communication systems are well known, the advantages of the spread spectrum phenomena have not heretofore been fully realized because of the limitations placed upon the receiver. In the invention of the aforementioned patent application, as well as in the present application, a narrow band communication system is provided in which signals from a narrow band signal generator module are converted to broad-band signals in the wide band modulator module and are radiated from a broad-band radiating antenna. This broad-band module is reciprocal in operation and therefore serves as both a modulator for the transmitted signals from the transmitting unit and serves as a demodulator for signals received from the antenna of the other communications unit, Spectrum broadening takes place in the modulator module of the transmitting unit and correlation takes place in the corresponding module of the receiving unit operating as a correlator. Spectrum broadening is applied to any uncoded electromagnetic wave supplied to either end of the modulator module. When a properly coded electromagnetic wave spectrum is applied to one end of the module and the correct code is applied as the modulation input, the resultant output signal from the module becomes the original modulated carrier which can be demodulated by a conventional AM or FM demodulator. In both of these systems the only components of the unit which need be broad-band are the modulator module and the antenna. Consequently, conventional AM or FM communication units can be used with the modulator correlator modules.
Whereas in the system of the aforesaid patent application the extent of the spread of the spectrum is relied upon to provide the privacy, in the present invention the PSK and PWM modulation is utilized to provide a modulated carrier signal in which the signal power spectrum is varied along the phase or frequency axis due to angle or phase modulation while the transmitted carrier amplitude remains constant, thus vastly increasing the ability to discretely address particular receiver terminals, provide communications privacy and minimize communication interference.